1. Field of the Invention
This invention pertains to the field of fire detection devices, and in particular to those devices which detect the particles of combustion usually found in the ambient environment as smoke.
2. Description of the Prior Art
A fire detection device measures some parameter of the ambient conditions present in an environment and is designed to react to a change in the ambient conditions caused by the event of a fire, such as the presence of the molecular particles of combustion or the wavelength of light emitted by a flame.
Some fire detection devices use an ionization principle in which an electric current passing through the air is attenuated by the presence of heavy particles combustion and the change in current is detected by a detector, causing an alarm to sound. Other fire detection devices use a photoelectric principle in which the passage of light through the ambient air is impeded by the presence of particles of combustion and either scattered light or the simulation of another light beam is detected, either case causing an alarm to be sounded. A third type of fire detection device uses the electromagnetic radiation emitted by a fire to trip a photoelectric detection device based on the wavelength of the light emitted by the fire.
Fire detection devices using the ionization principle detect the presence of particles of combustion within an ionization chamber, which is built around a small radioactive source which bombards with ionization particles, whether alpha or beta particles. Consequently, the ambient air becomes ionized, thus having an unbalanced electric charge. These ions will migrate through the air and constitute a minute electric current of about fifty to one hundred picoamperes. As smoke particles enter the ionization chamber through a general Brownian motion, the particles, being larger and more massive, tend to aglomerate onto the ions. Having more mass and therefore more inertia, these particles do not move as readily through the air. Consequently, this lack of movement of the ions creates a drop in the current through the chamber, this drop is translated into a change in voltage across the chamber which is amplified through an ultra high input impedance amplifier. Ionization detectors have several shortcomings. The first is that the ionization chamber is sensitive to particles of a fairly small size range. It will not detect particles of combustion smaller than or larger than its size range. The ionization chamber depends on the probability that at some time in the development of a fire from an incipient noncombustion hotspot to a raging fire, the fire will at some time produce particles of combustion of the size which the ionization chamber can detect. This size range is usually from 0.1/micron to one (1.0) micron. The ionization chamber is also susceptible to false alarms due to aerosols being injected into the air. Ammonia and synthetic cleaning agents are known for their ability to set off ionization detectors. Ionization dectectors are also affected by the velocity of air passing through the ionization chamber which can cause the ionic particles to be blown away faster than they are generated. While modern ionization detectors have been designed to minimize these problems, the problems still remain.
The photoelectric detectors also have some shortcomings. They are usually sensitive only to larger particles which are produced in later stages of combustion. Hence, they are slower to respond than the ionization type detectors. They also suffer from reliability problems. And it is necessary to keep all ambient light out of the photo-labyrinth. The labyrinth is heavily occluded and the passage of air within it is impeded. Hence, the photoelectric detector will be slow to respond to a combustion situation.
Detectors using electromagnetic radiation also have problems. These detectors sense ultraviolet or infrared radiation emitted from a fire and use this radiation to trip a detector. However, they are limited by the sensitivity of the photodetecting devices available. There are many spurious sources of infrared radiation and some of ultraviolet radiation. These devices are typically less sensitive than others and more prone to false alarms.
Of all the prior art detectors, the ionization chamber is the most sensitive. Even in its most sophisticated embodiments, it still suffers from two shortcomings. It requires a radioactive material and it cannot be used in an environment in which the ambient air must be moved rapidly.
The object of the present invention is to provide a fire detection device which is as sensitive as the ionization chamber, which does not use a radioactive material, which can be used effectively when the ambient air is moving, which does not have electronic design problems and which is safe and inexpensive.